1. Field of Invention
This invention concerns a process for preparing high melt flow polymers of alpha-olefins containing at least 3 carbon atoms, to copolymers thereof and to certain copolymer compositions which are believed to be new. The process, the operating conditions and physical properties of these polymers will be described fully hereinafter in the specification.
2. DESCRIPTION OF PRIOR ART
Alpha-olefin polymers, specifically those of propylene have been prepared heretofore in normally liquid hydrocarbon diluents employing what has become known to the art as Natta-Ziegler catalysts. High molecular weight polypropylenes of inherent viscosities ranging from about 1.5 to 18 as measured in decalin at 135.degree.C have heretofore been prepared by these techniques, specifically those wherein a titanium trihalide and an aluminum alkyl was used as a catalyst. The propylene polymers of inherent viscosities of about 5 and higher are usually of very high molecular weights and in order to be able to extrude or mold such polymers, it is usually required that the molecular weight be reduced for greater operational efficiency. An excellent means for accomplishing this reduction in molecular weight is disclosed in Vandenburg U.S. Pat. No. 3,051,690 of Aug. 28, 1962, which describes in some detail a method for employing hydrogen to control molecular weight of alpha-olefin polymers such as ethylene, propylene and higher. The patent discloses that polymers from ethylene and up to dodecene-1 and including branched chain alpha-olefins, as well as copolymers of the foregoing can be prepared using hydrogen to control the molecular weight while using normally liquid-hydrocarbon diluents such as heptane or hexane and as catalysts the hydrocarbon insoluble reaction product formed by contacting titanium tetrachloride with an aluminum alkyl compound and further activating this with an aluminum alkyl. The reaction pressures disclosed in the examples of this patent are nominal pressures, that is, below about 100 psig, even though higher or lower pressures, according to the specification, can be used. With respect to hydrogen pressures employed in propylene polymerizations, the examples indicate low pressures, and the preparation of propylene polymers of reduced specific viscosities (RSV) of from 3.7 to 0.8 for the heptane-insoluble fraction.
In most polymerization reactions wherein it is desired to control the molecular weight of the alpha-olefin polymer formed, it is indeed satisfactory to employ low hydrogen pressures, especially propylene polymerizations, since adequate control of molecular weight can be obtained, as illustrated in the above patent. Where high hydrogen pressures are used in propylene polymerizations wherein normally liquid hydrocarbons are used as diluents in the polymerization reactions, the prior art reports a decrease in polymerization rates (see British Pat. No. 908,101). It is not surprising therefore that even the Vandenberg patent admonishes against the use of high hydrogen pressures, for example pressures not to exceed about 185 psig (200 psia), since according to the teachings of that patent, extensive hydrogenation of the monomer can occur at these high pressures. The low conversion rates obtained by employing hydrogen as a molecular weight control agent where normally liquid hydrocarbon diluents are employed in these prior art reactions are also illustrated in the various examples of this patent showing conversion of propylene to polypropylene. Hydrogenation of the polymerizable monomer would readily result in low polymerization rates as might be expressed in pounds of polymer produced per pound of catalyst per hour.
In order to further illustrate the state of the prior art concerning the use of high hydrogen pressures, British patent 908,101, cited above, is discussed in more detail here since it strikingly illustrates that the use of too much hydrogen or too high a hydrogen pressure results in reduced polymerization rates In Example 5 of that specification it is illustrated for example that where an autoclave is pressurized with 7.0 psi hydrogen and a polymerization of propylene is carried out in dry petroleum ether there results a yield of 99 grams of polypropylene during a 28 hour reaction period. The catalyst disclosed for this example is a titanium trichloride aluminum chloride complex activated with diethylaluminum chloride. Thus, the rate of polymerization to produce a polymer having a melt viscosity (at 190.degree.C) of 1.9 .times. 10.sup.3 poises is somewhat less than 4 grams of polymer per gram of catalyst per hour. The statement in this specification, therefore, that high hydrogen pressures result in reduced rates and yields of polymer is substantially verified.
From the foregoing general state of the art, it may be concluded, therefore, that high hydrogen pressures in the polymerization of propylene and higher alpha-olefins results in reduced polymerization rates and low polymer yields and/or hydrogenation of the monomer at high hydrogen pressures leading to the same result. High melt flow propylene polymers, for example, higher than those illustrated in Example 5 of British Pat. No. 908,101 would seem difficult, if not substantially impossible to prepare at an economical rate in view of the teachings of the art. It is not surprising, therefore, in view of the above, that in U.S. Pat. No. 2,835,659, May 20, 1958, there is described a method for preparing extremely high melt flow polymers of propylene by a technique other than a direct polymerization reaction. This patent disclosed that ". . . it was not possible to polymerize propylene to form waxy polymers directly, since the major part of the low molecular weight polypropylene having a molecular weight in the range of 1,000 to 8,000 was an oily or a rubbery semi-solid similar to the polymers prepared using either Friedel-Crafts or free radical catalysts." The process for preparing waxy polymers, therefore, according to the patent is to employ a prepolymerized high molecular weight isotactic polypropylene (prepared by using a catalyst such as disclosed in Belgian Pat. No. 538,782) and heat it at 300.degree. to 450.degree.C in the absence of air to result in a wax characterized by having an inherent viscosity in tetralin at 145.degree.C of 0.5 or less. The polypropylene waxes prepared according to this technique possess crystallinities equal to or greater than those of the original high molecular weight polymer employed in the process according to this teaching. The new waxes are indicated as useful for coatings or for the preparation of emulsifiable polypropylene waxes according to U.S. Pat. No. 2,828,296.